Actually, I think it's a near-universal human coping mechanism.
I mean, I was taught to share, and love, and care for others, but not in a codependent way. I developed those coping mechanisms as I found myself watch my now XAH spiral into the pit that is alcoholism, but I didn't realize I was doing it.
It took me years to extricate myself, but learning that I was doing THE SAME THINGS that others did in the same situation was fascinating. And for the record, I come from a pretty strong, intact family. (I probably would have gotten out a long time prior, but my career sort of got in the way)
I'm free now, and don't miss the drama at all, so I think that codependency is a wide range of reactions and experiences.